Louvers are widely used in the walls of commercial, industrial, and institutional buildings at openings provided for various purposes, the most common purposes by far being for intake and exhaust of air for ventilation and for cooling of equipment. Often, several intake or exhaust openings are located in a cluster, such as at the exterior wall of an equipment room or floor. In high-rise buildings, equipment rooms are provided on certain floors and serve several adjacent floors above and below. In low-rise or one-story buildings, protected equipment enclosures may be provided on the roof, or one or more equipment rooms may be provided on a floor as required. In both instances, the openings at the building wall to ducts or to equipment spaces are often clustered.
The main function of a louver is to remove rain or snow ("moisture") from an air stream passing from the outside through an opening into a duct or space covered by the louver. The degree to which a louver is effective in removing moisture is a function of several aspects of the design of the louver, such as the slope and pitch distance of the blades, the configuration of the blades, the depth (the dimension perpendicular to the wall), and the blade orientation. In general, the more effective a louver is in removing moisture, the greater is the pressure drop across it. Inasmuch as it is desirable to minimize the pressure drop through a louver in order to avoid a waste of energy in drawing air in or pushing air out, the choice of a louver installation involves to some extent a compromise between effectiveness of moisture exclusion and energy efficiency.
Architects and building owners are also keenly interested in the appearance of the buildings they design and own. An element of the appearance of a building is the treatment of louvers. Uniformity of appearance is usually a design objective, one that very strongly favors horizontal blade louvers. Attaining uniformity of appearance often conflicts with both effectiveness of water removal and energy efficiency in several ways. First, for any given energy efficiency, vertical blade louvers are significantly more effective in removing moisture than are horizontal blade louvers, but vertical blade louvers change in appearance, depending on the vantage point of the viewer. Second, removal of moisture at an exhaust opening is significantly less difficult, because the air flow repels rather than induces moisture intake, and it is often less important to remove moisture at an exhaust opening than it is at an intake opening, but uniformity of appearance often leads the architect and owner to use the same louvers for both intake and exhaust openings. Third, worst case probable wind conditions used in design suggest the use of different louvers in different walls, depending on prevailing winds, another conflict with uniformity of appearance.
Previously known louvers are often custom-designed and built for particular installations, albeit based on standard blade and frame configurations. Each louver is a stand-alone unit and is separately fastened to elements of the building framing system, often in a manner that makes removal difficult at best. Custom design and manufacture increases costs. Permanent installation in the building wall makes repair or replacement of the louver difficult and expensive.